A semiconductor laser module comprises a semiconductor laser diode for emitting light of a predetermined wavelength, an optical fiber for transmitting the light to a following stage, and a rod lens for coupling the light emitted from the laser diode to the input end of the optical fiber, and is used, for example, as a light source in an optical communication system. In such a semiconductor laser module, the following characteristics are required
(1) An output light of a semiconductor laser diode is effectively coupled to an optical fiber. PA1 (2) The coupling efficiency of the output light does not fluctuate dependent on such factors as the change of temperature, the deterioration of time (secular variation), the vibration of the module, the mechanical impact applied to the module. PA1 (3) The semiconductor laser diode and other elements are air-tight sealed to stand a predetermined airproof test. PA1 (4) Light is not reflected from proximate ends of the rod lens, the optical fiber and so on back to the semiconductor laser diode.
Although an optical fiber communication technology has been primarily applied to a trunk line system of a communication network at an initial stage, the technology is now expanded to be applied to a branch line systems such as a subscriber system. In such an application, the semiconductor laser module is required to have a higher performance in various properties.
One type of a conventional semiconductor laser module has been proposed in the Japanese Utility Model provisional publication (Kokai) No. 62-157171. In the semiconductor laser module, a semiconductor laser diode is mounted on a stem similar in shape to a can-case. The stem is fixed to a lens holder having a rod lens in the inside thereof. The lens holder is fixed at its tip portion to a ferrule for protecting an optical fiber by providing a slide ring between them. For this structure, an output light of the semiconductor laser is coupled to the optical fiber. The semiconductor laser module will be explained in more detail later.
According to the conventional semiconductor module, however, there are disadvantages that bonding wire are difficult to connect to the laser diode, and the length of the bonding wires becomes long, because the laser diode is mounted on the inner bottom surface of the can-case. There are further disadvantages in conventional semiconductor module designs. High speed operation is difficult to carry out, because bonding wires are long. The precision of a temperature measurement of the laser diode is lowered, because a thermistor is provided behind the bottom of the can-case. There is a still further disadvantage that the alignment of the laser diode and the rod lens is difficult on a light axis of the output light, because the laser diode is mounted at a predetermined position on the bottom of the can-case which is orthogonal to the light axis.
Next, a structure in which the laser diode is mounted on the stem will be discussed. In general, a semiconductor chip is first soldered to a chip carrier. Then, the semiconductor chip is screened and selected in regard to its properties. Finally, the semiconductor chip mounted on the chip carrier is fixed to the stem by use of a solder which is lower in a melting point than a solder for the chip carrier. For instance, a solder of PbSn (63/37) having a melting point of 183.degree. C. is used for the fixing of a chip carrier having a laser diode thereon, where the laser diode is mounted on the chip carrier by use of a solder of AuSn (80/20) having a melting temperature of 283.degree. C., so that the softening of the AuSn solder does not occur to avoid the deviation of the laser diode on the chip carrier after the fixing of the chip carrier.
According to the conventional semiconductor laser module, however, there is a disadvantage that a high reliability of an optical system is not obtained, because a plasticized deformation occurs in the PbSn solder due to a solder creep phenomenon, where a specified preserving temperature of the semiconductor laser module is set to be 85.degree. C. That is, the solder creep phenomenon is observed under a constant load at a temperature which is much lower than a melting point of the solder.